1. Field of the Invention
This invention relates to a process for producing a porous film having flexibility and a structure of uniform fine pores and hence superior water vapor permeability and water resistance and useful for waterproof clothings, waterproof covers, packaging materials, etc.
2. Description of the Related Art
There has so far been known a process for producing a porous film by blending non-compatible substances such as inorganic fine powders with a polyolefin resin in a specified proportion, followed by melt-molding the resulting blend into a film or sheet and then uniaxially or biaxially stretching the film or sheet.
However, such a process has had a drawback that since the resin is stretched and oriented by stretch processing, the film or sheet increases in the hardness so that its flexibility is damaged.
In order to overcome such a problem, the following processes for producing a porous film have been proposed:
(1) a process of blending a thermoplastic elastomer with a polyolefin resin and a filler (Japanese patent application laid-open No. Sho 59-30833/1984); PA1 (3) a process of blending barium sulfate as an inorganic fine powder with a polyolefin resin (G.B. No. 2,151,538). PA1 A: very soft PA1 B: somewhat soft PA1 C: considerably hard
(2) a process of blending a liquid or waxy hydrocarbon polymer with a polyolefin resin and a filler (U.S. Pat. No. 4,472,328); and
However, porous films obtained according to these production processes have the following practical drawbacks:
According to the process (1), the resulting porosity is insufficient and the water vapor permeability is inferior. According to the process (2), the resulting porous film has a problem that in the high temperature atmosphere or after a long time lapse, hydrocarbon polymers bleed out on the surface of the resulting film so that the surface is sticky. According to the process (3), the resulting porous film has a good flexibility and sufficient water vapor permeability, but the stretching stability i.e. the high stretchability is inferior. Further, as a problem in common to these processes, it is impossible to produce an extremely thin film of about 20.mu.. Furthermore, another process of blending a third component brings about a large increase in cost.